Allogeneic hematopoietic cell transplant (HCT) is a widely used therapy for hematologic malignancy. It is important to give the option of HCT to patients without an HLA-matched donor; this is most acute for ethnic minorities. Haploidentical HCT may offer the best or only chance for cure but often fails due to persistant malignancy or fatal toxicity. It is important to improve disease control without increased toxicity of the regimen. We have shown that CD45 radioimmunotherapy (RIT) delivers radiation to leukemic sites, sparing normal organs. We hypothesize haploidentical HCT may be enhanced via CD45 RIT by maximizing delivery to target tissue with minimal toxicity by reducing total-body irradiation (TBI) and chemotherapy. We performed studies to find the maximal intensity of anti-leukemic RIT combined with haploidentical HCT while preserving graft-versus-leukemia effects. We established an HCT regimen in B6SJLF1/J mice (H-2Db haplotype) conditioned with fludarabine (FLU, days -6 to -2), followed by TBI (250, 500, 750 cGy; day -1). Mice received 15 million donor (CB6F1/J, H-2Dd) BM cells (day 0), then cyclophosphamide (CY) for graft-versus-host disease (GvHD) prophylaxis (day +2). Chimerism was TBI dose-dependent; mice receiving ≥500 cGy were fully chimeric 4 weeks post-HCT, persisting ≥8 months. We then treated B6SJLF1/J mice with or without FLU and 200-400 μCi of 90Y-labeled anti-murine CD45 Ab (30F11, day -3), followed by 15 million donor BM cells (day 0) and CY (day +2). All mice receiving 200 or 300 μCi 90Y-CD45 Ab survived 30 days after HCT with no evidence of toxicity or GvHD. Four of 10 mice receiving 400 μCi 90Y-CD45 Ab died by day 25 due to toxicity. At day 25 blood was taken to detect chimerism via flow cytometry. Three of 5 mice that received 200 μCi 90Y-CD45 Ab and FLU showed chimerism with 23-77% CD8+ T-cells expressing H-2Dd. Four of 5 (80%) mice receiving 300 μCi 90Y-30F11 and 2/3 (67%) receiving 400 μCi 90Y-30F11, in the presence of FLU, were chimeric with 22-89% and 25-64% CD8+ donor cells, respectively. Mice receiving 200, 300, or 400 μCi 90Y-CD45 Ab without FLU were chimeric with 10-71% donor cells in 4/5 (80%), 2/5 (40%) and 1/4 (25%) mice, respectively. These data suggest anti-CD45 RIT may be a well-tolerated regimen allowing engraftment of haploidentical marrow without TBI and chemotherapy. Current studies testing pretargeted RIT may improve targeting of malignant CD45+ cells with reduced TBI and FLU, and increased therapeutic index.

PURPOSE Radioimmunotherapy may improve the outcome of hematopoietic cell transplantation for hematologic malignancies by delivering targeted radiation to hematopoietic organs while relatively sparing nontarget organs. We evaluated the organ localization of yttrium-90-labeled anti-CD45 ((90)Y-anti-CD45) antibody in macaques, a model that had previously predicted iodine-131-labeled anti-CD45 ((131)I-anti-CD45) antibody biodistribution in humans. EXPERIMENTAL DESIGN Twelve Macaca nemestrina primates received anti-CD45 antibody labeled with 1 to 2 mCi of (90)Y followed by serial blood sampling and marrow and lymph node biopsies, and necropsy. The content of (90)Y per gram of tissue was determined by liquid scintillation spectrometry. Time-activity curves were constructed using average isotope concentrations in each tissue at measured time points to yield the fractional residence time and estimate radiation absorbed doses for each organ per unit of administered activity. The biodistribution of (90)Y-anti-CD45 antibody was then compared with that previously obtained with (131)I-anti-CD45 antibody in macaques. RESULTS The spleen received 2,120, marrow 1,060, and lymph nodes 315 cGy/mCi of (90)Y injected. The liver and lungs were the nontarget organs receiving the highest radiation absorbed doses (440 and 285 cGy/mCi, respectively). Yttrium-90-labeled anti-CD45 antibody delivered 2.5- and 3.7-fold more radiation to marrow than to liver and lungs, respectively. The ratios previously observed with (131)I-anti-CD45 antibody were 2.5-and 2.2-fold more radiation to marrow than to liver and lungs, respectively. CONCLUSIONS This study shows that (90)Y-anti-CD45 antibody can deliver relatively selective radiation to hematopoietic tissues, with similar ratios of radiation delivered to target versus nontarget organs, as compared with the (131)I immunoconjugate in the same animal model.

Background. Using a canine model of nonmyeloablative hematopoietic cell transplantation (HCT), the authors demonstrated that pretransplant radioimmunotherapy with the &agr;-emitter bismuth-213 (213Bi) coupled to anti-CD45 or anti–T-cell receptor &agr;&bgr; (TCR&agr;&bgr;) monoclonal antibodies (mAb), together with postgrafting immunosuppression with mycophenolate mofetil (MMF) and cyclosporine A (CsA), achieved stable engraftment of dog leukocyte antigen (DLA)-identical marrow. Engraftment was achieved with doses of 3.6 to 8.8 mCi/kg 213Bi, but signs of liver toxicity were noted in all dogs. To find a safe and effective dose for further trials, the authors performed a dose deescalation study in 15 dogs with 2.7 to 0.8 mCi/kg 213Bi. Methods. 213Bi was linked to the mAb using the metal-binding chelate CHX-A″–DTPA. All dogs received three to six injections of 213Bi linked to anti-CD45 or anti-TCR&agr;&bgr; mAb followed by marrow grafts from DLA-identical littermates and postgrafting MMF and CsA. Results. During follow-up of greater than 30 weeks, engraftment remained stable in all evaluable dogs conditioned with 1.4 to 2.1 mCi/kg 213Bi–anti-CD45 or 2.0 to 2.7 mCi/kg 213Bi–anti-TCR&agr;&bgr;. Only one dog conditioned with 1.5 mCi/kg 213Bi–anti-TCR&agr;&bgr; had stable engraftment, whereas two rejected their grafts. In both groups, all dogs conditioned with less than 1.3 mCi/kg 213Bi rejected their grafts. No signs of graft-versus-host disease or other toxicities were noted. Only mild and transient elevation of liver function tests occurred in 4 of 15 dogs. Conclusions. This study demonstrates that dose deescalation of radioimmunotherapy with 213Bi labeled to anti-CD45 or anti-TCR&agr;&bgr; as conditioning for nonmyeloablative HCT minimizes toxicity without compromising engraftment. With a dose of 2 mCi/kg 213Bi, further trials using radioimmunotherapy with 213Bi for nonmyeloablative HCT seem feasible.

The in vivo application of the protein streptavidin is limited by its propensity to localize to kidney, particularly when it is used as a carrier of radionuclides in Targeted Radionuclide Therapy. Our previous studies demonstrated that modification of recombinant "core" streptavidin (rSAv) by reaction of lysine residues with succinic anhydride and arginine residues with 1,2-cyclohexanedione dramatically decreases the kidney concentrations over that obtained with wild-type rSAv. In this investigation, we explored the role of lysine and arginine residues in kidney localization further by evaluating site-directed mutants of rSAv. In the five mutants studied, the four lysine residues found in each subunit of rSAv were replaced (independently) with an alanine (K80A, K121A, K132A, K134A), and a specific arginine was replaced with a histidine (R59H). The rSAv mutants were prepared from a "core" rSAv construct produced by expression in E. coli that had 124 amino acids (residues 13-136). Another rSAv construct that had 127 amino acids (residues 13-139), used in most of our previous studies, was also included for comparison. As an additional comparison, succinylated rSAv was prepared and evaluated. The rSAv proteins were radioiodinated and injected into athymic mice that were on a biotin-free diet for 5-7 days prior, and biodistribution data were obtained (for most proteins) at 1, 4, 24, and 48 h postinjection. The data obtained show large differences in kidney localizations of the wild-type rSAv and some rSAv mutants. The largest difference in the kidney concentration was noted for the rSAv-K134A mutant (1.90 +/- 0.22%ID/g; 24 h pi) as compared to the wild-type rSAv (31.83 +/-5.26%ID/g) at the same time point. The concentration of rSAv-K134A mutant in kidney was slightly lower than that obtained with succinylated rSAv. At the 24 h time point, the kidney concentrations of the rSAv-R59H mutant (8.95 +/- 2.94%ID/g) and the rSAv-K121A mutant (11.86 +/- 1.61%ID/g) were lower than wild-type rSAv, but the rSAv mutants rSAv-K80A (27.95 +/- 1.82%ID/g) and rSAv-K132A (32.50 +/- 10.09%ID/g) were essentially the same. The data suggests that specific lysine and arginine residues are involved in kidney localization. Possible mechanisms for the observed kidney localization are discussed.

One pretargeting approach to cancer radioimmunotherapy utilizes an antibody-streptavidin conjugate that is first localized to the tumor. A "clearing agent" is then administered to remove the excess bioconjugate from blood, followed by injection of the radiolabeled biotin therapeutic. In this study, the role of streptavidin-biotin affinity in this pretargeting system was investigated for the first time in vivo, with a reduced affinity, site-directed streptavidin mutant and with radiolabeled bis-biotin reagents. The S45A streptavidin mutant (SA-S45A), which displays a faster off-rate for biotin, was utilized with a bivalent biotin carrier that retains high avidity for the streptavidin mutant. Mice were fed either a normal or biotin-deficient diet, yielding serum endogenous biotin concentrations of 31 nM and 5 nM, respectively. Lymphoma-bearing nude mice pretargeted with 1F5 Antibody-SA-Wild Type (WT) bioconjugates produced (125)I-bis-biotin tumor concentrations of 2.2%ID/g and 7.0%ID/g in mice fed normal diets vs biotin-deficient diets. (125)I-bis-biotin tumor concentrations of mice pretargeted with 1F5-SA-S45A were 12%ID/g and 10%ID/g for mice fed normal and biotin-deficient diets, respectively. However, poor clearance of the 1F5-SA-S45A with the biotinylated clearing agent led to high normal organ concentrations of (125)I-bis-biotin. A galactosylated human serum albumin (HSA) modified with bis-biotin was then tested, and normal organ (125)I-bis-biotin concentrations were significantly reduced. Tumor-to-organ ratios achieved for 1F5-SA-S45A with the HSA-bis-biotin clearing agent in mice with high serum biotin were similar to those achieved with 1F5-SA-WT in mice with low serum biotin. These results demonstrate that exchange of bound endogenous biotin with lower affinity streptavidin mutants is possible, and that corresponding use of bis-biotin carriers can nearly eliminate the differences in therapeutic radioactivity at the tumor site in animals on normal vs biotin-deficient diets. The results also interestingly demonstrate, however, that improved clearance agents capable of removing the lower affinity streptavidin-antibody conjugate are needed to achieve comparable specificity in tumor to blood or normal organ ratios.

Iodination and radioiodination reactions of several anionic nido- and closo-monocarbon carboranes were conducted. Iodinations occurred more rapidly with nido-carboranes than with closo-carboranes. The most rapid iodination and radioiodination reactions occurred with unsubstituted carboranes. C-amino and C-ammonium derivatives did not iodinate under the conditions studied. Both nido- and closo-carboranes with C-NH-acetyl and C-NH-succinyl substituents iodinated, but the nido-carboranes iodinated under milder reaction conditions. Biodistributions of nido-1-succinylamido-[131I]carborane and closo-1-succinylamido-[125I]carborane were similar in mice, but blood clearance of the nido- compound was slower.

An investigation has been conducted to prepare and evaluate several radiohalogenated biotin derivatives as part of our studies to develop reagents for carrying (211)At in cancer pretargeting protocols. The primary goal of the investigation was to determine the in vivo stability and distribution properties of astatinated biotin derivatives. In addition to astatination, the biotin derivatives were radioiodinated for in vitro and in vivo comparison. Biodistributions were conducted in athymic mice, with sacrifice times of 1, 4, and 24 h to correspond to 9%, 32%, and 90% of (211)At decay (t(1/2) = 7.21 h). In the investigation, two biotin derivatives, 1a and 2a, were synthesized which had structures that contain a biotin moiety, a biotinidase-blocking moiety, an ether linker moiety, and an aryl stannane moiety for radiohalogenation. Biotin derivatives 1a and 2a were radiolabeled with (125/131)I to give [(125)/(131)I]1b or [(125)I]2b and with (211)At to give [(211)At]1c or [(211)At]2c. In vivo studies demonstrated that co-injected [(125)I]2b and [(131)I]1b had very similar tissue distributions in athymic mice. Co-injection of [(211)At]2c and [(125)I]2b provided data that indicated that rapid deastatination occurred in vivo. A second set of biotin derivatives, 3a, 4a, and 5a, were synthesized which had structures that contain a biotin moiety, a biotinidase-blocking moiety, and an anionic nido-carborane moiety for radiohalogenation. The biotin derivatives 4a and 5a contained an aryl moiety not present in 3a, and 5a had a trialkylamine functionality not present in 3a or 4a. Biotin derivative 3a was radioiodinated, but was not further investigated. Biotin derivatives 4a and 5a were radiolabeled with (211)At and (125)I to produce [(125)I]4b/[(211)At]4c and [(125)I]5b/[(211)At]5c. Comparison of [(125)I]4b and (separately) [(125)I]5b with [(131)I]1b showed that the nido-carborane containing biotin derivatives were retained in blood and tissue more than the aryl iodide derivative. In vivo evaluations of [(211)At]4c/[(125)I]4b and (separately) [(211)At]5c/[(125)I]5b indicated that some deastatination occurred in these compounds, but it was much less than observed for the aryl derivative [(211)At]2c. While the nido-carborane containing biotin derivatives provide a significant improvement in astatine stability over biotin derivatives previously studied, additional derivatives need to be prepared and studied to further improve the in vivo stability and blood/tissue clearance of these compounds.

Two major immunologic barriers, the host-versus-graft (HVG) and graft-versus-host (GVH) reactions, have to be overcome for successful allogeneic hematopoietic cell transplantation. T cells were shown to be primarily involved in these barriers in the major histocompatibility complex identical setting. We hypothesized that selective ablation of T cells using radioimmunotherapy together with postgrafting immunosuppression would suffice to ensure stable allogeneic engraftment. We had described a canine model of nonmyeloablative marrow transplantation in which host immune reactions were impaired by a single dose of 200 cGy total body irradiation (TBI), and both GVH and residual HVG reactions were controlled by postgrafting immunosuppression with mycophenolate mofetil (MMF) and cyclosporine (CSP). Here, we substituted the alpha-emitter bismuth-213 (213Bi) linked to a monoclonal antibody (mAb) against T-cell receptor (TCR) alphabeta, using the metal-binding chelate diethylenetriaminepentaacetic acid (DTPA) derivative cyclohexyl-(CHX)-A", for 200 cGy TBI. Biodistribution studies using a gamma-emitting indium-111-labeled anti-TCRalphabeta mAb showed uptake primarily in blood, marrow, lymph nodes, spleen, and liver. Four dogs were treated with 0.13 to 0.46 mg/kg TCRalphabeta mAb labeled with 3.7 to 5.6 mCi/kg (137-207 MBq/kg) 213Bi. The treatment was administered in 6 injections on days -3 and -2 followed by transplantation of dog leukocyte antigen-identical marrow on day 0 and postgrafting immunosuppression with MMF/CSP. The therapy was well tolerated except for elevations of transaminases that were transient in all but one of the dogs. No other organ toxicities or signs of graft-versus-host disease were noted. The dogs had prompt allogeneic hematopoietic engraftment and achieved stable mixed donor-host hematopoietic chimerism with donor contributions ranging from 5% to 55% after more than 30 weeks of follow up.

An investigation has been conducted to assess the in vivo stability of a series of astatinated benzamides and astatinated nido-carborane compounds in mice. It was hypothesized that the higher bond strength of boron-astatine bonds in the nido-carboranes might provide increased stability toward in vivo deastatination. Four tri-n-butylstannylbenzamides were prepared for radiohalogenation and evaluation in vivo. Those compounds were N-propyl-4-(tri-n-butylstannyl)benzamide 1a, N-propyl-3-(tri-n-butylstannyl)benzamide 2a, ethyl 4-tri-n-butylstannylhippurate 3a, and 4-tri-n-butylstannyl-hippuric acid 4a. Seven mono-nido-carboranyl derivatives were prepared for radiohalogenation and in vivo evaluation. Four of the seven mono-carboranyl derivatives (5a, 6a, 7a, 13a) contained a 3-(nido-carboranyl)propionamide functionality, and the remaining compounds (8a, 8g, 10a) contained a 4-(nido-carboranyl)aniline functionality. Two additional derivatives (11a, 12a) were prepared that contained bis-(nido-carboranylmethyl)benzene moieties (also referred to as Venus flytrap complexes (VFCs). All benzamide and nido-carborane compounds underwent facile iodination and radiohalogenation, except a 4-(nido-carboranyl)aniline derivative, 8a. Iodination of 8a resulted in a mixture, of which the desired iodinated product was a minor component. Therefore, radiohalogenation was not attempted. It is believed that the mixture of products is due to the presence of a thiourea bond. Previous studies have shown that thiourea bonds can interfere with halogenation reactions. In vivo comparisons of the compounds were conducted by co-injection of dual labeled (125/131I and 211At) compounds. Tissue distribution data were obtained at 1 and 4 h postinjection of the radiolabeled compounds, as that was sufficient to determine if astatine was being released. Stability of the astatinated compound was assessed by the difference in concentration of radioiodine and astatine in lung and spleen. All of the benzamides were found to undergo rapid deastatination in vivo. The nido-carborane derivatives appeared to be slightly more stable to in vivo deastatination; however, they had long blood residence times. The surprising finding was that the VFC derivatives did not release 211At in vivo, even though they rapidly localized to liver. This finding provides encouragement that stable conjugates of 211At may be attained if appropriate modifications of the VFC can be made to redirect their excretion through the renal system.

UNLABELLED:Pretargeted radioimmunotherapy (RIT) using streptavidin (sAv)-conjugated antibodies before radiolabeled-biotin is a promising approach to improve absorbed dose ratios and achieve high durable remission rates with diminished systemic toxicity. This study compared the immunoscintigraphy, toxicity, and therapeutic efficacy of pretargeted RIT with conventional RIT using an anti-CD20 antibody.METHODS:Athymic mice bearing Ramos human Burkitt's lymphoma xenografts were injected intraperitoneally with a 1F5-sAv conjugate followed 24 h later by a galactosylated, biotinylated clearing agent (CA) and, finally, 3 h later by (111)In- or (90)Y-labeled 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA)-biotin. The comparison groups consisted of mice injected with conventional, directly labeled (111)In- or (90)Y-1F5.RESULTS:Rapid tumor uptake of radioactivity within 2 h was observed with the pretargeting approach, resulting in high-contrast tumor images at 24 h with minimal blood-pool radioactivity. Although conventional radiolabeled antibodies produced clear tumor images at 24 h, a large amount of radioactivity was present in the blood pool. The tumor-to-blood ratio was 3.5:1 with pretargeting compared with 0.4:1 with conventional (111)In-1F5. Pretargeted RIT with 29.6 MBq (800 micro Ci) (90)Y-DOTA-biotin cured 100% of mice with tolerable toxicity, whereas conventional RIT with (90)Y-1F5 at a dose of 14.8 MBq (400 micro Ci) produced no cures, induced profound pancytopenia, and was lethal to all mice.CONCLUSION:These results suggest that anti-CD20 pretargeted RIT may be superior to conventional radiolabeled antibodies in terms of radioimmunoscintigraphy, toxicity, and therapeutic efficacy for treatment of B-cell lymphomas.

Recombinant streptavidin (rSAv) is of interest as a carrier of alpha-emitting radionuclides in pretargeting protocols for cancer therapy. Due to the inherently high kidney localization of rSAv, modification of this protein is required before it can be useful in pretargeting. Previous studies (Wilbur, D. S., Hamlin, D. K. et al. (1998) Bioconjugate Chem. 9, 322-330) have shown that succinylation of rSAv using succinic anhydride decreases the kidney localization appreciably. In continuing studies, the biotin binding characteristics and biodistribution in mice of rSAv modified by reaction with succinic anhydride (amine modification) or 1,2-cyclohexanedione (arginine modification) have been compared. Modification of rSAv was conducted using 5-50 mol equiv of succinic anhydride and 60-200 mol equiv of 1,2-cyclohexanedione. Most studies were conducted using rSAv modified with the highest quantities of reagents. Succinylation of rSAv did not alter binding with biotin derivatives, but a small increase in the biotin derivative dissociation rate was noted for arginine-modified rSAv. Amino acid analysis of 1,2-cyclohexanedione-treated rSAv indicated about 40% of the arginine residues, or an average of 1.6 residues per subunit, were modified, whereas none of the lysine residues were modified. IEF analyses showed that the pI of the arginine-modified rSAv was 5.3-6, whereas the pI for the succinylated rSAv was approximately 4. Electrospray mass spectral analyses indicated that one to three conjugates of 1,2-cyclohexanedione, and two to three conjugates of succinic anhydride, were obtained per subunit. Both modification reactions resulted in greatly decreasing the kidney localization of rSAv (normally 20-25% ID/g at 4, 24, and 48 h pi). However, the kidney concentration for the succinylated rSAv continued to decrease (5% ID/g to 1.5% ID/g) from 4 to 48 h pi, whereas the concentration (5% ID/g) remained constant over that period of time for the arginine-modified rSAv. In contrast to this, the liver concentration appeared to be slightly higher (3% ID/g vs 2% ID/g) at the later time points for the succinylated rSAv. When less than 50 mol equiv of succinic anhydride were employed in the modification of rSAv, a correlation between increasing kidney localization with decreasing equivalents reacted was observed. Although the differences in the two modified rSAv are not substantial, succinylated rSAv appears to have more favorable properties for pretargeting studies.